docs/topics/impala_truncate_table.xml - impala - Git at Google

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 <!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
 <concept rev="2.3.0" id="truncate_table">

   <title>TRUNCATE TABLE Statement (<keyword keyref="impala23"/> or higher only)</title>
   <titlealts audience="PDF"><navtitle>TRUNCATE TABLE</navtitle></titlealts>
   <prolog>
     <metadata>
       <data name="Category" value="Impala"/>
       <data name="Category" value="SQL"/>
       <data name="Category" value="DDL"/>
       <data name="Category" value="S3"/>
       <data name="Category" value="Tables"/>
       <data name="Category" value="Disk Storage"/>
       <data name="Category" value="Developers"/>
       <data name="Category" value="Data Analysts"/>
     </metadata>
   </prolog>

   <conbody>
     <p rev="2.3.0">
       <indexterm audience="hidden">TRUNCATE TABLE statement</indexterm> Removes
       the data from an Impala table while leaving the table itself. </p>
     <p conref="../shared/impala_common.xml#common/syntax_blurb"/>
     <codeblock>TRUNCATE [TABLE] <ph rev="2.5.0 IMPALA-2641">[IF EXISTS]</ph> [<varname>db_name</varname>.]<varname>table_name</varname></codeblock>
     <p conref="../shared/impala_common.xml#common/ddl_blurb"/>
     <p conref="../shared/impala_common.xml#common/usage_notes_blurb"/>
     <p> Often used to empty tables that are used during ETL cycles, after the
       data has been copied to another table for the next stage of processing.
       This statement is a low-overhead alternative to dropping and recreating
       the table, or using <codeph>INSERT OVERWRITE</codeph> to replace the data
       during the next ETL cycle. </p>
     <p> This statement removes all the data and associated data files in the
       table. It can remove data files from internal tables, external tables,
       partitioned tables, and tables mapped to HBase or the Amazon Simple
       Storage Service (S3). The data removal applies to the entire table,
       including all partitions of a partitioned table. </p>
     <p> Any statistics produced by the <codeph>COMPUTE STATS</codeph> statement
       are reset when the data is removed. </p>
     <p> Make sure that you are in the correct database before truncating a
       table, either by issuing a <codeph>USE</codeph> statement first or by
       using a fully qualified name
           <codeph><varname>db_name</varname>.<varname>table_name</varname></codeph>. </p>
     <p>The optional <codeph>TABLE</codeph> keyword does not affect the behavior
       of the statement.</p>
     <p rev="2.5.0 IMPALA-2641"> The optional <codeph>IF EXISTS</codeph> clause
       makes the statement succeed whether or not the table exists. If the table
       does exist, it is truncated; if it does not exist, the statement has no
       effect. This capability is useful in standardized setup scripts that are
       might be run both before and after some of the tables exist. This clause
       is available in <keyword keyref="impala25_full"/> and higher. </p>
     <p conref="../shared/impala_common.xml#common/disk_space_blurb"/>
     <p conref="../shared/impala_common.xml#common/s3_blurb"/>
     <p rev="2.2.0"> Although Impala cannot write new data to a table stored in
       the Amazon S3 filesystem, the <codeph>TRUNCATE TABLE</codeph> statement
       can remove data files from S3. See <xref href="impala_s3.xml#s3"/> for
       details about working with S3 tables. </p>
     <p conref="../shared/impala_common.xml#common/cancel_blurb_no"/>
     <p conref="../shared/impala_common.xml#common/permissions_blurb"/>
     <p rev=""> The user ID that the <cmdname>impalad</cmdname> daemon runs
       under, typically the <codeph>impala</codeph> user, must have write
       permission for all the files and directories that make up the table. </p>
     <p conref="../shared/impala_common.xml#common/kudu_blurb"/>
     <p conref="../shared/impala_common.xml#common/kudu_no_truncate_table"/>
     <p conref="../shared/impala_common.xml#common/example_blurb"/>
     <p> The following example shows a table containing some data and with table
       and column statistics. After the <codeph>TRUNCATE TABLE</codeph>
       statement, the data is removed and the statistics are reset. </p>
     <codeblock>CREATE TABLE truncate_demo (x INT);
 INSERT INTO truncate_demo VALUES (1), (2), (4), (8);
 SELECT COUNT(*) FROM truncate_demo;
 +----------+
 | count(*) |
 +----------+
 | 4        |
 +----------+
 COMPUTE STATS truncate_demo;
 +-----------------------------------------+
 | summary                                 |
 +-----------------------------------------+
 | Updated 1 partition(s) and 1 column(s). |
 +-----------------------------------------+
 SHOW TABLE STATS truncate_demo;
 +-------+--------+------+--------------+-------------------+--------+-------------------+
 | #Rows | #Files | Size | Bytes Cached | Cache Replication | Format | Incremental stats |
 +-------+--------+------+--------------+-------------------+--------+-------------------+
 | 4     | 1      | 8B   | NOT CACHED   | NOT CACHED        | TEXT   | false             |
 +-------+--------+------+--------------+-------------------+--------+-------------------+
 SHOW COLUMN STATS truncate_demo;
 +--------+------+------------------+--------+----------+----------+
 | Column | Type | #Distinct Values | #Nulls | Max Size | Avg Size |
 +--------+------+------------------+--------+----------+----------+
 | x      | INT  | 4                | -1     | 4        | 4        |
 +--------+------+------------------+--------+----------+----------+

 -- After this statement, the data and the table/column stats will be gone.
 TRUNCATE TABLE truncate_demo;

 SELECT COUNT(*) FROM truncate_demo;
 +----------+
 | count(*) |
 +----------+
 | 0        |
 +----------+
 SHOW TABLE STATS truncate_demo;
 +-------+--------+------+--------------+-------------------+--------+-------------------+
 | #Rows | #Files | Size | Bytes Cached | Cache Replication | Format | Incremental stats |
 +-------+--------+------+--------------+-------------------+--------+-------------------+
 | -1    | 0      | 0B   | NOT CACHED   | NOT CACHED        | TEXT   | false             |
 +-------+--------+------+--------------+-------------------+--------+-------------------+
 SHOW COLUMN STATS truncate_demo;
 +--------+------+------------------+--------+----------+----------+
 | Column | Type | #Distinct Values | #Nulls | Max Size | Avg Size |
 +--------+------+------------------+--------+----------+----------+
 | x      | INT  | -1               | -1     | 4        | 4        |
 +--------+------+------------------+--------+----------+----------+
 </codeblock>
     <p> The following example shows how the <codeph>IF EXISTS</codeph> clause
       allows the <codeph>TRUNCATE TABLE</codeph> statement to be run without
       error whether or not the table exists: </p>
     <codeblock rev="2.5.0 IMPALA-2641">CREATE TABLE staging_table1 (x INT, s STRING);
 Fetched 0 row(s) in 0.33s

 SHOW TABLES LIKE 'staging*';
 +----------------+
 | name           |
 +----------------+
 | staging_table1 |
 +----------------+
 Fetched 1 row(s) in 0.25s

 -- Our ETL process involves removing all data from several staging tables
 -- even though some might be already dropped, or not created yet.

 TRUNCATE TABLE IF EXISTS staging_table1;
 Fetched 0 row(s) in 5.04s

 TRUNCATE TABLE IF EXISTS staging_table2;
 Fetched 0 row(s) in 0.25s

 TRUNCATE TABLE IF EXISTS staging_table3;
 Fetched 0 row(s) in 0.25s
 </codeblock>
     <p conref="../shared/impala_common.xml#common/related_info"/>
     <p>
       <xref href="impala_tables.xml#tables"/>, <xref
         href="impala_alter_table.xml#alter_table"/>, <xref
         href="impala_create_table.xml#create_table"/>, <xref
         href="impala_partitioning.xml#partitioning"/>, <xref
         href="impala_tables.xml#internal_tables"/>, <xref
         href="impala_tables.xml#external_tables"/>
     </p>
   </conbody>
 </concept>
	<?xml version="1.0" encoding="UTF-8"?><!--
	Licensed to the Apache Software Foundation (ASF) under one
	or more contributor license agreements. See the NOTICE file
	distributed with this work for additional information
	regarding copyright ownership. The ASF licenses this file
	to you under the Apache License, Version 2.0 (the
	"License"); you may not use this file except in compliance
	with the License. You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing,
	software distributed under the License is distributed on an
	"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
	KIND, either express or implied. See the License for the
	specific language governing permissions and limitations
	under the License.
	-->
	<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
	<concept rev="2.3.0" id="truncate_table">

	<title>TRUNCATE TABLE Statement (<keyword keyref="impala23"/> or higher only)</title>
	<titlealts audience="PDF"><navtitle>TRUNCATE TABLE</navtitle></titlealts>
	<prolog>
	<metadata>
	<data name="Category" value="Impala"/>
	<data name="Category" value="SQL"/>
	<data name="Category" value="DDL"/>
	<data name="Category" value="S3"/>
	<data name="Category" value="Tables"/>
	<data name="Category" value="Disk Storage"/>
	<data name="Category" value="Developers"/>
	<data name="Category" value="Data Analysts"/>
	</metadata>
	</prolog>

	<conbody>
	<p rev="2.3.0">
	<indexterm audience="hidden">TRUNCATE TABLE statement</indexterm> Removes
	the data from an Impala table while leaving the table itself. </p>
	<p conref="../shared/impala_common.xml#common/syntax_blurb"/>
	<codeblock>TRUNCATE [TABLE] <ph rev="2.5.0 IMPALA-2641">[IF EXISTS]</ph> [<varname>db_name</varname>.]<varname>table_name</varname></codeblock>
	<p conref="../shared/impala_common.xml#common/ddl_blurb"/>
	<p conref="../shared/impala_common.xml#common/usage_notes_blurb"/>
	<p> Often used to empty tables that are used during ETL cycles, after the
	data has been copied to another table for the next stage of processing.
	This statement is a low-overhead alternative to dropping and recreating
	the table, or using <codeph>INSERT OVERWRITE</codeph> to replace the data
	during the next ETL cycle. </p>
	<p> This statement removes all the data and associated data files in the
	table. It can remove data files from internal tables, external tables,
	partitioned tables, and tables mapped to HBase or the Amazon Simple
	Storage Service (S3). The data removal applies to the entire table,
	including all partitions of a partitioned table. </p>
	<p> Any statistics produced by the <codeph>COMPUTE STATS</codeph> statement
	are reset when the data is removed. </p>
	<p> Make sure that you are in the correct database before truncating a
	table, either by issuing a <codeph>USE</codeph> statement first or by
	using a fully qualified name
	<codeph><varname>db_name</varname>.<varname>table_name</varname></codeph>. </p>
	<p>The optional <codeph>TABLE</codeph> keyword does not affect the behavior
	of the statement.</p>
	<p rev="2.5.0 IMPALA-2641"> The optional <codeph>IF EXISTS</codeph> clause
	makes the statement succeed whether or not the table exists. If the table
	does exist, it is truncated; if it does not exist, the statement has no
	effect. This capability is useful in standardized setup scripts that are
	might be run both before and after some of the tables exist. This clause
	is available in <keyword keyref="impala25_full"/> and higher. </p>
	<p conref="../shared/impala_common.xml#common/disk_space_blurb"/>
	<p conref="../shared/impala_common.xml#common/s3_blurb"/>
	<p rev="2.2.0"> Although Impala cannot write new data to a table stored in
	the Amazon S3 filesystem, the <codeph>TRUNCATE TABLE</codeph> statement
	can remove data files from S3. See <xref href="impala_s3.xml#s3"/> for
	details about working with S3 tables. </p>
	<p conref="../shared/impala_common.xml#common/cancel_blurb_no"/>
	<p conref="../shared/impala_common.xml#common/permissions_blurb"/>
	<p rev=""> The user ID that the <cmdname>impalad</cmdname> daemon runs
	under, typically the <codeph>impala</codeph> user, must have write
	permission for all the files and directories that make up the table. </p>
	<p conref="../shared/impala_common.xml#common/kudu_blurb"/>
	<p conref="../shared/impala_common.xml#common/kudu_no_truncate_table"/>
	<p conref="../shared/impala_common.xml#common/example_blurb"/>
	<p> The following example shows a table containing some data and with table
	and column statistics. After the <codeph>TRUNCATE TABLE</codeph>
	statement, the data is removed and the statistics are reset. </p>
	<codeblock>CREATE TABLE truncate_demo (x INT);
	INSERT INTO truncate_demo VALUES (1), (2), (4), (8);
	SELECT COUNT(*) FROM truncate_demo;
	+----------+
	\| count(*) \|
	+----------+
	\| 4 \|
	+----------+
	COMPUTE STATS truncate_demo;
	+-----------------------------------------+
	\| summary \|
	+-----------------------------------------+
	\| Updated 1 partition(s) and 1 column(s). \|
	+-----------------------------------------+
	SHOW TABLE STATS truncate_demo;
	+-------+--------+------+--------------+-------------------+--------+-------------------+
	\| #Rows \| #Files \| Size \| Bytes Cached \| Cache Replication \| Format \| Incremental stats \|
	+-------+--------+------+--------------+-------------------+--------+-------------------+
	\| 4 \| 1 \| 8B \| NOT CACHED \| NOT CACHED \| TEXT \| false \|
	+-------+--------+------+--------------+-------------------+--------+-------------------+
	SHOW COLUMN STATS truncate_demo;
	+--------+------+------------------+--------+----------+----------+
	\| Column \| Type \| #Distinct Values \| #Nulls \| Max Size \| Avg Size \|
	+--------+------+------------------+--------+----------+----------+
	\| x \| INT \| 4 \| -1 \| 4 \| 4 \|
	+--------+------+------------------+--------+----------+----------+

	-- After this statement, the data and the table/column stats will be gone.
	TRUNCATE TABLE truncate_demo;

	SELECT COUNT(*) FROM truncate_demo;
	+----------+
	\| count(*) \|
	+----------+
	\| 0 \|
	+----------+
	SHOW TABLE STATS truncate_demo;
	+-------+--------+------+--------------+-------------------+--------+-------------------+
	\| #Rows \| #Files \| Size \| Bytes Cached \| Cache Replication \| Format \| Incremental stats \|
	+-------+--------+------+--------------+-------------------+--------+-------------------+
	\| -1 \| 0 \| 0B \| NOT CACHED \| NOT CACHED \| TEXT \| false \|
	+-------+--------+------+--------------+-------------------+--------+-------------------+
	SHOW COLUMN STATS truncate_demo;
	+--------+------+------------------+--------+----------+----------+
	\| Column \| Type \| #Distinct Values \| #Nulls \| Max Size \| Avg Size \|
	+--------+------+------------------+--------+----------+----------+
	\| x \| INT \| -1 \| -1 \| 4 \| 4 \|
	+--------+------+------------------+--------+----------+----------+
	</codeblock>
	<p> The following example shows how the <codeph>IF EXISTS</codeph> clause
	allows the <codeph>TRUNCATE TABLE</codeph> statement to be run without
	error whether or not the table exists: </p>
	<codeblock rev="2.5.0 IMPALA-2641">CREATE TABLE staging_table1 (x INT, s STRING);
	Fetched 0 row(s) in 0.33s

	SHOW TABLES LIKE 'staging*';
	+----------------+
	\| name \|
	+----------------+
	\| staging_table1 \|
	+----------------+
	Fetched 1 row(s) in 0.25s

	-- Our ETL process involves removing all data from several staging tables
	-- even though some might be already dropped, or not created yet.

	TRUNCATE TABLE IF EXISTS staging_table1;
	Fetched 0 row(s) in 5.04s

	TRUNCATE TABLE IF EXISTS staging_table2;
	Fetched 0 row(s) in 0.25s

	TRUNCATE TABLE IF EXISTS staging_table3;
	Fetched 0 row(s) in 0.25s
	</codeblock>
	<p conref="../shared/impala_common.xml#common/related_info"/>
	<p>
	<xref href="impala_tables.xml#tables"/>, <xref
	href="impala_alter_table.xml#alter_table"/>, <xref
	href="impala_create_table.xml#create_table"/>, <xref
	href="impala_partitioning.xml#partitioning"/>, <xref
	href="impala_tables.xml#internal_tables"/>, <xref
	href="impala_tables.xml#external_tables"/>
	</p>
	</conbody>
	</concept>